Mobile PC manufacturers compete to increase system performance while reducing or at least maintaining power consumption. Mobile PC performance has increased dramatically. However, because it is not desirable to have larger batteries, and battery efficiency has not kept pace with processor performance, battery life for systems operating at peak performance has been drastically reduced. Manufacturers introduced the capability of power and performance control to prolong battery life. For example, a user watching a movie may wish to lower power consumption at the cost of diminished quality in order to prolong battery life long enough to complete the movie. Power and performance control is also used to control thermals. For example if a processor is overheating, the user may lower the performance thus lowering the power consumption and thus reducing the heat. In a typical power management system (PMS) the user provides a series of inputs to the power management portion of the operating system (OS). Alternatively, the PMS might be an embedded part of the OS. The user might input a preference toward battery life or toward system performance. The user might indicate energy conservation for DC operation and system performance optimization for AC operation.
Historically, the reduction in power consumption had a linear relationship to the reduction in system performance. For example, a system running at 500 Mhz and using 10 watts could be throttled down to 250 Mhz and use 5 watts. When a system is run against a fixed workload, a PMS exhibiting this linear relationship provides little benefit in the way of prolonged battery life. That is, a system running at half the speed for twice as long will accomplish the same amount for the expended energy. The system will run cooler, but no more work is accomplished.
More recent systems address this concern by taking advantage of the equation governing power consumption in CMOS circuits. This equation is P=kV2F, where P is the power consumed, k is some constant, V is the applied voltage and F is the operating frequency. Application of this equation shows that a small reduction in voltage may provide a large reduction in power consumption. Using a voltage-varying scheme in which the power is applied over time, therefore, allows for fixed workload to be accomplished with less energy and hence prolonged battery life. A typical PMS would provide a high-voltage/high-frequency mode for AC use and a low-voltage/low-frequency mode for DC use. The modes are implemented by a software program which detects whether the AC adapter has been plugged in, or not, and switches mode accordingly. The user could also provide input to the system and, if desired, choose not to switch to low performance mode. The PMS software may be incorporated within the OS and indicates to an application and driver that the power source has changed, the driver then communicates with the firmware that switches modes.
Although such a PMS prolongs battery life, it does not address the issue of reduced performance. While on battery the system runs at a lower frequency and the user does not get the full benefit of system performance. If the user places the system into a high performance mode the battery life is diminished.